The manufacture of cans is at present commercially accomplished in a variety of ways. One of the more common methods is to roll a sheet of flat steel, ordinarily coated on both sides with a thin layer of tin, into a cylinder and weld up the resulting seam. Commonly the edges of the steel sheet are overlapped slightly to form the seam. This weld can be performed in a variety of ways. Possibly the most common is through resistance welding, where the can is passed between a pair of electrically energized rollers, thus causing a current to flow between the overlapped edges forming the seam. If pressure is applied to the rollers, they essentially flatten out the overlapping edges so that the cylinder thus formed is essentially uniform in wall thickness around its circumference. Ends may then be applied to the can by conventional techniques.
One form of welding machine which is of great utility in such welding processes is that marketed under the name "Soudronic." This machine is available in a variety of models. Essentially, it is a large electric motor driving a generator which produces an AC current of on the order of 2,000 amperes at 18 volts, which has, naturally, a pulsed waveform. This sort of waveform when applied to seam welding of cans produces a "stitching" effect whereby the weld is produced not by a continuous application of electrical energy and pressure, but by pulsed application of the electrical energy. Thus the weld, if examined microscopically, exhibits a long series of joined tiny welds.
It will be apparent that it is essential to the manufacture of a can which is leak-proof that the weld be of continuous and perfect integrity throughout its length. Furthermore, it will be appreciated that according to modern production methods, such cans in order to be economically manufactured must be made at very high rates of speed, on the order of 300 per minute. Therefore, the weld process must be developed to the point that it is reliable, efficient and susceptible of very high speed application. While the Soudronic machine is capable of welding cans in accordance with the above requirements, it will be appreciated that it is no simple matter to set the machine up to perform at such specifications. Furthermore, once the proper settings of the machine controls have been achieved, there remains the problem of testing the seams as they are manufactured since it is highly desirable to remove poorly welded cans from the production process before they go any further. Therefore, a need exists in the art for an improved weld monitor method which will detect defective welds at the weld point so that the cans do not have to be subjected to further leak tests at a later stage in the production line. Ideally, in a "stitching" weld application as described above every stitch would be monitored.